ASHRAE Journal - January 2012 - 32

of 57°F (14°C) will often allow space latent loads to be removed by a primary airﬂow rate similar to the space ventilation air requirement.

(For 100,000 ft2 Building in Atlanta)

Design Sensible Load and Ventilation Requirements

Perimeter Interior Perimeter Interior Conference Conference Ofﬁces Ofﬁces Rooms Rooms (27,000 ft2) (63,000 ft2) 2 2
(3,000 ft ) (7,000 ft )

Design Example

Average Zone Size (ft2)

900

450

2,000

450

An active chilled beam system is Number of Occupants 6 20 20 20 being used in a 100,000 ft 2 (9290 m 2) office building in Atlanta where deOccupant Density (ft2/person) 150 22.5 100 22.5 sign dry-bulb/MCWB temperatures Design Sensible Cooling Load (Btu/h·ft2) 40.0 52.5 11.0 23.5 are 92°F/74°F (33°C/23°C) and design wet-bulb/MCDB temperatures Design Latent Cooling (Btu/h·ft2) 1.3 8.9 2.0 8.9 are 85°F/77°F (31°C/26°C). The room design conditions are 75°F Occupant OA Rate (RP) (cfm/person) 5 5 5 5 (24°C) with a relative humidity not to Area OA Rate (RA) (cfm/ft2) 0.06 0.06 0.06 0.06 exceed 55%. Tables 1 and 2 deﬁne the building ocZone Ventilation Effectiveness (EZ) 0.90 cupancy as well as its sensible load design parameters. All latent inﬁltration Occupant Diversity Factor (D) 0.70 gains are assigned to the perimeter areas and are based on an inﬁltration rate of Table 1: Description of building and loads used in examples. 0.2 cfm/ft2 (0.09 L/s·m2) of façade. Ventilation rates are calculated according to ASHRAE Standard HVAC System Design Conditions 62.1-2010. Sensible Outdoor Design Conditions Chilled beams with a water-side sensible cooling rate of 34 Btu/h·cfm (6 Wh·m3) of primary air at an entering air to Dry-Bulb Temperature (°F) 92 mean water temperature differential of 16°F (9°C) are used MCWB Temperature (°F) 74 in all four scenarios. In each case, the chilled water supply temperature has been established at 1°F (0.6°C) above the Humidity Ratio (grains) 97 room dew-point temperature and the sensible cooling per3/lb ) 14.21 Speciﬁc Volume (ft m formance of the coil has been adjusted accordingly. Table 3 details four system scenarios that will be studied. Latent Outdoor Design Conditions All scenarios are based on local design wet-bulb conditions Wet-Bulb Temperature (°F) 77 of 82°F (28°C) dry bulb and 77°F (25°C) wet bulb. The airMCDB Temperature (°F) 82 handling unit for each scenario is illustrated in Figure 2. All air-handling units are equipped with total energy wheels with Humidity Ratio (grains) 130 83% sensible and 68% total efﬁciency. 14.06 Speciﬁc Volume (ft3/lbm) Scenario A is the baseline, which includes a conventional air-handling unit with a chilled water cooling and dehumidiChilled Beam Performance Characteristics ﬁcation coil. A primary air mixture of outdoor and return air Maximum Primary Airﬂow Rate (cfm/linear ft) 15 conditioned to a 50°F (10°C) dew point is delivered at 55°F Coil Sensible Heat Removal (Btu/h·cfm) (13°C) to spaces maintained at 75°F (24°C) with design rela(At a chilled water ﬂow rate of 1.0 gpm 34 tive humidity levels of 50%. and EAT– MWT = 16°F) Scenario B1 assumes a dedicated outdoor air system (DOAS) air-handling unit with a passive desiccant wheel sup- Note: All air-handling units are provided with total energy recovery wheels with 83% sensible efﬁciency, 68% total efﬁciency. plying 100% outside air to the building. Scenario B2 uses the same air-handling unit but delivers a Table 2: Design conditions and chilled beam characteristics. primary air mixture of return and outdoor air. Both scenarios Table 4 allows comparison of the primary and outside airB1 and B2 involve delivery of primary air to the room at 68°F (20°C) dry bulb and a 47°F (8.3°C) dew-point tem- ﬂow rates and water ﬂow rates required for space ventilation, perature to spaces designed for 75°F (24°C) and 50% rela- cooling and dehumidiﬁcation under each design scenario. It also estimates the energy use of the system at the local design tive humidity. Scenario C uses the same air-handling unit as Scenario wet-bulb conditions. The upper section of Table 4 summarizes the various zone A, but the space relative humidity design has been relaxed to airﬂow requirements for each scenario. In all cases, the mini55% (design temperature remains the same).
32 ASHRAE Journal ashrae.org January 2012
ASHRAE Journal - January 2012

Table of Contents for the Digital Edition of ASHRAE Journal - January 2012
